Printing of Responsive Photonic Cellulose Nanocrystal Microfilm Arrays

Abstract

Interactive materials capable of changing appearance upon exposure to external stimuli, such as photonic inks, are generally difficult to achieve on a large scale as they often require self‐assembly processes that are difficult to control macroscopically. Here this problem is overcome by preparing arrays of cellulose nanocrystal (CNC) microfilms from discrete nanoliter sessile droplets. The obtained microfilms show extremely uniform and intense color, enabling exceptional consistency in optical appearance across the entire array. The color can be controlled through the initial ink formulation, enabling the printing of polychromatic dot‐matrix images. Moreover, the high surface‐to‐volume ratio of the microfilms and the intrinsic hydrophilicity of the natural building block allow for a dramatic real‐time colorimetric response to changes in relative humidity. The printed CNC microfilm arrays overcome the existing issues of scalability, optical uniformity, and material efficiency, which have held back the adoption of CNC‐based photonic materials in cosmetics, interactive‐pigments, or anticounterfeit applications.

Angular optical response of cellulose nanocrystal films explained by the distortion of the arrested suspension upon drying

“…The alignment of individual CNCs and of their cholesteric domains can be controlled in suspension using various methods such as external fields, [23,[53][54][55][56][57][58][59][60][61], shear, [21,[62][63][64][65] or anchoring. [20,22,49] When the suspension is exposed to a sufficient magnetic field prior to the kinetic arrest transition, the CNCs align with their long axis perpendicular to the field. However, the field required to align individually these anisotropic diamagnetic crystals is usually difficult to access (typically µ 0 H ≥ 20 T for micron-long cellulose crystals).…”

“…[47] Controlling them can lead to an isotropic, [48] or an anisotropic compression to the cholesteric structure. [8,49,50] This significantly influences the order and the morphology of the dried material, as well as the resulting angular optical response. [48,51] We recently highlighted this key mechanism in the assembly of CNCs in microdroplets.…”

From Equilibrium Liquid Crystal Formation and Kinetic Arrest to Photonic Bandgap Films Using Suspensions of Cellulose Nanocrystals

“…This can be retained during evaporation of the water until a solid cellulose film arises that displays vivid structural colors when observed under ambient white light. This important observation [3] was warmly welcomed, because the potential of an abundantly available bioresource for the development of materials with optical functionality (photonic bandgap) was recognized, suggesting a plethora of applications, from biobased iridescent pigments that might be suitable for food and cosmetic applications [4,5] to templates for inorganic materials with complex internal structure [6], covert encryption [7], sensors of humidity [8,9] or pressure [10] and much more. Beyond the beauty of these films and their potential use in various photonic devices, the helically arranged CNCs are also considered as a basis for high-performance composite materials that show excellent mechanical properties coupled with low weight [11], mimicking the helically modulated structures of nature's top performers, as in crustacean shells or exoskeletons of beetles [12,13] (where the structure is formed by chitin, the 'relative' of cellulose in the animal kingdom).…”

Flexible and Structural Coloured Composite Films from Cellulose Nanocrystals/Hydroxypropyl Cellulose Lyotropic Suspensions

“…Moreover, when 3.3 wt.% of HPC is introduced in the colloidal suspension, a reduction of 4 μm in the pitch value is observed (comparing with the system with 2.2 wt.% of HPC); however, this value is still a higher value than the one observed for the neat CNCs colloidal suspension. The structural colouration observed in the CNCs films has been related to several parameters that influence the colloidal suspension as, for example: CNCs concentration; ionic strength [3]; energy given with ultrasonic treatment [9]; exposure to magnetic field [10] or electric field [11] and the drying rate of the solvent [12] combined with planar anchoring [13]. Although these interesting studies allow us to obtain iridescent CNCs films, due to their crystalline nature, they present low mechanical strength (highly brittle), which can restrict their range of application.…”